As minimum dimensions continue to shrink in the fabrication of digital integrated circuits, corresponding improvements in the unity gain threshold frequency (f.sub.t), maximum oscillation frequency (f.sub.max), and propagation delay (.tau..sub.pd) of transistors are increasingly difficult to realize. This is so because in general the parasitic elements associated with the transistors do not scale in proportion to the minimum dimensions. That is, two and three dimensional effects begin to become dominant.
In the case of the bipolar junction transistor (BJT), key parasitics which must be minimized to improve switching speed are base width, base resistance and base-collector capacitance. The minimization of these parasitics, unfortunately, tend to have a deleterious effect on the common emitter forward current gain (h.sub.fe) and the collector to emitter breakdown voltage (BV.sub.CEO) As the base width, for example, is made more narrow, the base doping must be increased in order to maintain BV.sub.CEO. This reduces the time the device can be exposed to high temperatures during subsequent processing as well as degrading h.sub.fe. A practical BJT, therefore, is fundamentally limited to an f.sub.t of about 20-30 GHz. This limit is set by the fact that when the base doping reaches the 10.sup.18 /cm.sup.3 regime, trap-assisted tunnelling in the base seriously degrades h.sub.fe. The integrated base charge, on the other hand, which is required to maintain BV.sub.CEO at about 3-5 V then prevents the base width from being narrowed any further for a practical device.
To achieve further improvements in speed, the use of the heterojunction bipolar transistor (HBT) has been proposed The h.sub.fe of the HBT is not strongly dependent upon the respective base and emitter dopings, but rather by the difference in band widths (the band gap) between the emitter and base. The most common HBT's are fabricated using the III-V semiconductor compounds such as, for example, GaAs and AlGaAs. These, on the other hand, have not achieved the integration levels of silicon (Si) devices and are further limited by wafer diameter, material strength, substrate cost and a relatively high level of intrinsic defects.
Recently, HBT's have been fabricated using a Si emitter and a pseudomorphic Ge.sub.x Si.sub.1-x strained-layer base. The presence of germanium (Ge) in the base reduces the band-gap in proportion to the amount of Ge present. Devices of 40 GHz f.sub.t have been fabricated with devices capable of 60 GHz f.sub.t predicted. The current gain of most HBT devices can in principle be maintained with high base doping and, therefore, comparatively small base resistance. The latter improves f.sub.max and delays the onset of emitter de-biasing. Further, because current crowding in the emitter is reduced, the device may be able to sustain higher currents before base conductivity modulation and the Kirk Effect manifest themselves.
The Ge.sub.x Si.sub.1-x base HBT is subject to a physical limitation because the covalent radius of Ge is larger than that of Si. A small lattice mismatch, therefore, exists between the two layers at each Si-Ge.sub.x Si.sub.1-x interface. For a thin enough film of Ge.sub.x Si.sub.1-x on a Si substrate the mismatch is accommodated by a compressive stress in the Ge.sub.x Si.sub.1-x film. When the thickness exceeds a critical thickness, a high density of the misfit dislocations appear at the interface along with threading dislocations which extend from the interface to the surface. These dislocations will give rise to junction leakage particularly when decorated with heavy metal contaminants. Once misfit dislocations appear, the Ge.sub.x Si.sub.1-x layer becomes nearly strain free since the stress is relieved by the plastic deformation at the interface. The critical thickness for the onset of misfit dislocations is inversely proportional to the concentration of Ge in the film. When using pure Ge the thickness of the film which can be achieved on a Si substrate before plastic deformation occurs is on the order of 1-3 nm. Thus, the realization of a pure Ge base on a Si collector has not been possible in the past.
Therefore, a need has arisen for a process for manufacturing a substantially pure and narrow epitaxial Ge base in a Si-based HBT. Such a layer is needed to continue to improve the switching speed bipolar transistors.